Anatomy of the Human Eye

Rocky Young, Ph.D.

Professor of Ophthalmology & Visual Sciences

  1. General organization
    1. Layers of the eyeball
    1. Outer layer: Cornea (1/6, transparent) & Sclera (5/6, opaque)
    2. Middle layer (Uveal tract): Choroid, Ciliary body, & Iris
    3. Inner layer: Retinal pigment epithelium & sensory retina
    1. Compartments of the eyeball
    1. Aqueous compartment (anterior and posterior chambers)
    2. Lens
    3. Vitreous compartment
    1. Exterior anatomy
    1. 1.Conjunctiva -- vascular lining on the anterior surface of eyeball (excluding the cornea).
    2. 2.Anterior ciliary arteries enter eye just behind corneosclera border (limbus)
    3. 3.Vortex veins exit just behind of equator of the eyeball
    4. 4.Extraocular muscles attached to eyeball: 4 recti & 2 obliques
    5. 5.Ciliary nerves and posterior ciliary arteries (enter eye around optic nerve)
    6. 6.Optic nerve (emerges from nasal side and below posterior pole)
    7. 7.Central retinal artery & vein (enter eyeball with optic nerve)
  1. Outer layer of the eye
    1. Cornea is the major (optical) refractive element of the eye.
      1. Corneal curvature & index of refraction account for optical power.
      2. Transparency attributed to no blood vessels, relatively few cells, & the arrangement of collagen fibers in mucopolysaccharide matrix.
      3. Main layers: Epithelium (5-6 cells thick, provides barrier between external and intraocular environment), stroma (90% of thickness, composed of collagen fibers), endothelium (maintains relative dehydration of corneal stroma).
      4. Tear film layer produces "optical smoothness": Oily layer (from meibomian gland), aqueous layer (from lacrimal gland), & mucoid layer (from goblet cells).
      5. Cornea has pain reception (ciliary n).
      6. Oxygen & nutrition: Air & tears (anteriorly), aqueous (posteriorly), and limbal circulation (around circumference).
    1. Sclera is the "white" of the eye.
      1. Sclera itself is dense, fibrous tissue. Like cornea, it is composed of collagen fibers. Water content, arrangement of diameter of collagen fibers, and type of mucopolysaccharides all contribute to opacity.
      2. Sclera is covered by other tissues, e.g., conjunctiva & episclera.
      3. Has pain reception (ciliary n)
      4. Lamina cribosa = sieve-like structure in the sclera through which optic nerve fibers exit the eyeball.
    1. Limbus = the transitional region from cornea and sclera. Located here are:
      1. Blood vessels and lymphatics.
      2. Trabecular network and canal of Schlemm, involved in the drainage of the aqueous humor.
  1. Middle layer of the eye ( "uveal tract")
    1. Iris = a diaphragm whose aperture (the pupil) changes size.
      1. Two layers, stroma and the epithelium.
      1. Stroma consists of connective tissue, blood vessels, & sphincter muscle. Amount of melanin here determines the color of the iris. If only slight amounts, light reflections from the pigmented epithelium causes scattering and thus a blue color.
      2. Epithelium: (i) Myoepithelium consists of dilator muscle. (ii) Pigment layer has cells packed with melanin.
      1. Sphincter muscle is a 1-mm band of smooth muscle that encircles the pupil. When it contracts, the pupil constricts. N III (parasympathetic) via ciliary ganglion to short ciliary n.
      2. Dilator muscle runs radially. When it contracts, pupil dilates. Sympathetic N via ciliary ganglion to ciliary n.
      3. The iris separates the anterior and posterior chambers of the aqueous compartment.
    1. Ciliary body = ring of tissue located between the iris and choroid. Ciliary body is divided into epithelial (inner) and uveal (outer) portions.
      1. Epithelial portion produces aqueous humor. Two parts of the epithelial portion are the pars plicata ("folds") and the pars plana ("featureless").
      1. Pars plicata has some 60-70 folds (the "ciliary processes").
      2. Pars plana is the portion closest to choroid.
      1. Uveal portion contains vascular supply and ciliary muscles (involved in accommodation). Contraction of the muscles makes the lens more convex (more optical power).
      2. Zonules (zonules of Zinn) = fibers holding the lens to the ciliary body.
      3. Ora ("border") serrata = border between ciliary body and choroid. Retina abruptly ends here. Has serrated margin.
    1. Choroid =posterior portion of the uveal tract; is a highly vascular tissue.
    1. Suprachoroid layer --- loosely attaches choroid to sclera.
    2. Layers of blood vessels:
      1. Large vessels (Haller's layer): mostly veins, empties into vortex v.
      2. Medium vessels (Sattler's layer): mostly veins, empties into vortex v.
      3. Choriocapillaris: Supplies retinal pigment epithelium & outer retina.
    1. Bruch's membrane layer -- transition between choriocapillaris & retinal pigment epithelium.
    2. Macroscopic appearance is black due to pigmentation.
    3. Choroid also contains nerves and arteries to anterior portion of eye.
  1. Inner layer of the eye
    1. Retinal pigment epithelium: Single layer of (pigmented) cells supporting metabolic activity of photoreceptors. Microvilli at the apical surface "embrace" photoreceptor outer segments. Remember choriocapillaris at the basal surface of RPE.
    2. Sensory retina --- multiple layers:
      1. Three layers of cells: Photoreceptor cell, inner nuclear (horizontal, bipolar & amacrine cells), & ganglion cell layers.
      2. Two synaptic layers: Outer plexiform and inner plexiform.
      3. Skeletal support: Mueller cells (traversing retinal thickness) & glia cells (nerve fiber layer).
      4. Other layers: Nerve fiber layer, inner & outer limiting membranes.
    1. Ophthalmoscopic landmarks: Fovea, macula, optic disk, nerve fiber layer, ora serrata, retinal vascular arcade, macular pigment.
    2. Optical transparency of inner retinal layers: Pigmentation, retinal blood vessels, and lack of myelination.
  1. Aqueous humor maintains the intraocular pressure in the eye. Supplies oxygen & metabolites to (as well as removes waste from) avascular structures.
    1. Aqueous is secreted by "ciliary processs" (see pars plicata).
    2. It flows from the posterior chamber through the pupil into the anterior chamber.
    3. It drains through the trabecular network, passes through the canal of Schlemm, and exits into the episcleral veins.
  1. Lens is a transparent biconvex body which provides variable optical power (i.e., allows the eye to focus at different distances).
    1. Anatomy: Lies behind the iris. Has capsule, cortex, and nucleus.
    2. Attachment in eye: Capsule attached to the ciliary body by zonules (zonule of Zinn).
    3. Accommodation: Contraction of ciliary muscles (parasympath III n) leads to increase in lens curvature.
    4. Aging effect: New fibers are formed under the capsule around the circumference and pushed to the center of lens. Zones or bands of discontinuity in the cross section of a lens delineate tissue age.
  1. Vitreous body is a transparent, viscous fluid or gel-like structure composed of a network of collagen fibers suspended in a liquid containing hyaluronic acid.
    1. Vitreous is of no importance in maintaining the shape of the eye, but the physical relationship between the vitreous gel and the retina is the basis of serious eye problems.
    2. Points of attachment in the eye: (1) Annular zone of adhesion straddling the ora serrata, (2) adhesion between gel and posterior lens capsule, (3) adhesion to the retina via the "inner limiting membrane".